Contact annunciator with acknowledgment and master alarm relay



March 12, 1963 R. F. WERNLUND 3,081,449

. CONTACT ANNUNCIATOR WITH ACKNOWLEDGMENT AND MASTER ALARM RELAY FiledFeb. 19. 1959 4 Sheets-Sheet 1 RN H 9% w v u v I N959 @N MN \SQQDW H. LI v km N N% 7 ,3? QM RE A A A W1 A J4 l AMAV NA QR AN A i R 4Sheets-Sheet 2 R. F. WERNLUND AND MASTER ALARM RELAY CONTACT ANNUNCIATORWITH ACKNOWLEDGMENT March 12, 1963 Filed Feb. 19. 1959 March 12, 1963 R.F. WERNLUND CONTACT ANNUNCIATOR WITH ACKNOWLEDGMENT AND MASTER ALARMRELAY 4 Sheets-Sheet 3 Filed Feb. 19, 1959 March 12, 1963 R. F. WERNLUND3,081,449

CONTACT ANNUNCIATOR WITH ACKNOWLEDGMENT AND MASTER ALARM RELAY FiledFeb. 19. 1959 4 Sheets-Sheet 4 VOLTAGE SUPPLY I I I United States PatentRobertshaw-Fulton Controls Company, Richmond, Va., a corporation ofDelaware Filed Feb. 19, 1959, Scr. No. 794,408 13 Claims. (Cl. 340213)This invention relates to electronic control systems, and moreparticularly to electrical circuitry and components for monitoring thecondition of a group of variables and indicating the presence of analarm condition.

In certain industrial processes, it is necessary to monitor a largenumber of variables. Such monitoring is often accomplished by individualcondition responsive units which continuously compare some physicalparameter such as temperature or pressure with a preset value and derivetherefrom the electrical signal related to the magnitude of anydeviation which is discovered. Such prior art scanning systems areusually designed to produce continuously variable voltages.

The present invention contemplates a simple and inexpensive On-Oifsystem for indicating the presence of alarm conditions anywhere in agiven installation. With the present invention, there is provided analarm device which visibly or audibly indicates the closure or openingof any one of a number of individual alarm contacts provided at remotepoints in an istallation. In the several embodiments of the inventiondisclosed herein, alarm conditions are signaled by the opening orclosing of a set of condition responsive contacts at points remote fromthe sampling device. The types of conditions which the invention iscapable of monitoring may vary Widely. For instance, with the circuitryand components of the present system, the alarm contacts which initiatean audible or visual alarm indication may be actuated in response toexcessive vibration in a turbine-generator or other type prime mover.Or, dangerously high liquid levels or temperatures may be used toinitiate the closure of contacts and initiate the observable alarmindication provided in the present invention. The invention lends itselfequally well to the function of announcing the occurrence of dangerousvelocities or pressures of fluid flow in a conduit. Many other types ofalarm condition, such as the over-speeding of a rotary machine may beemployed for the purpose of actuating the alarm contacts utilized ingenerating ob servable alarm indications by means of controlledtransient electric voltages in the manner disclosed and claimed herein.

Accordingly, therefore, an object of the present invention is a simpleOn-Oif scanning system for indicating alarm conditions in aninstallation.

Another object of the present invention is to render a gaseous dischargetube conductive in response to alarm conditions for initiating anelectrical transient condition suitable for actuating audible or visualalarm indicators.

A further object of the present invention is to render nonconductive inresponse to alarm conditions a normally conductive space dischargedevice for actuating relay circuitry which actuates an alarm.

These and other objects and advantages of the present invention willbecome apparent by reference to the accompanying detailed descriptionand drawings, in which like numerals indicate like parts, and in which:

FIG. 1 shows diagrammatically one embodiment of the present invention;

FIG. 2 is similar to FIG. 1 and shows ment of this invention;

FIG. 3 is similar to FIG. 1 and shows still another embodiment of thisinvention;

FIGS. 4A, 4B and 40 show various wave forms of the another embodi-Patented Mar. 12, 1963 pulse appearing at the grid of the spacedischarge device used in the various embodiments of this invention; and

FIG. 5 is similar tov FIG. 1 and shows still another embodiment of thisinvention.

Turning now to the detailed description of the invention, and moreparticularly'to the system shown in FIG. 1 of the accompanying drawings,the numeral 10 has been used to indicate generally the circuitry andcomponents of one embodiment of the invention. In this circuit, there isprovided a voltage supply 12 with a grounded positive terminal, and anegative terminal which is conductively connected to one end of adropping resistor 14. The dropping resistor 14 effectively prevents anytendency of the associated circuitry to oscillate in the manner of aring type multivibrator.

The opposite end of dropping resistor 14 is connected to a common bus16, shown immediately to the right of the voltage supply. In the bank ofindividual circuits connected to the common bus 16, the uppermostindividual circuit is seen to include an alarm lamp 18A of the gaseoustype. The alarm lamp 18A may comprise any suitably high hysteresisdevice such as a neon bulb or the like which has the characteristic ofbecoming ionized at a given potential and thereafter being maintainedconductive by a value of potential lower than that of the ionizingpotential. v

The alarm lamp 18A is connected in series with one terminal of anormally open alarm contact 20A. The opposite, terminal of the alarmcontact 20A is connected to ground through the resistor 22A. Thejunction point between the alarm contact'20A and the resistor 22A isconnected to one plate of a capacitor 24A. The opposite plate of thecapacitor, in turn, is secured to a common bus 26.

The parallel bank of individual circuits connected in FIG. 1 between thecommon bus conductors 16 and 26 will be seen to comprise a group ofseries circuits, each substantially identical to the uppermost circuitdescribed immediately above. Thus, alarm lamp 183 is connected fromcommon bus 16 to common bus 26 by normally open alarm contacts 20B andcapacitor 24B, with the shunt resistor 22B connected to ground betweenthe alarm contacts and the capacitor. In like manner, alarm lamp 18C, isconnected from common bus 16 to common bus 26 by means of the normallyopen alarm contacts 20C and the capacitor 24C, with the juncture betweenthe alarm contacts and the capacitor connected to ground throughresistor 220. It will be appreciated that the several alarm contacts arestrategically positioned throughout the installation, at the site of theenvironments sought to be monitored.

To the right of the elements described immediately above, the numeral 28has been used to identify a conventional plural element space dischargedevice, such as a triode or the like. The cathode of the device 28 isconnected to ground and the control grid element is connected directlyto the common bus 26. It will be noted that the grid element is alsoconnected to ground through a grid resistor 30. a V

The anode element of the discharge device 28 is connected to a voltagesupply 32 through a relay operating coil F. The coil F is mounted tocontrol a set of normally open contacts F shown immediately to the upperright thereof. Discharge device 28 is normally conductive during normalor non-alarm conditions and thus the contacts F will ordinarily beclosed.

To the right of contacts F the reference numeral 34 has been used todesignate a momentary contact push button switch which may be springbiased to a normally open position. This switch is depressed :by theoperator to acknowledge alarms which have occurred in the system, and toreset the circuitry to detect further alarm conditions. In practice,depressing the switch 34 permits current to flow from the voltage supply32 through the relay operating coil G connected thereto. This has theeffect of energizing and closing a pair of normally open contacts GSince contacts F are usually closed during non-alarm conditions, it willbe evident that coil G is able to maintain itself in energized conditionby means of contacts F and G after the push button switch 34 has beenrestored to open position.

The relay operating coil G also controls a pair of normally closedcontacts G The contacts G control the alarm circuitry 36, which in turnacts to energize or deencrgize the alarm horn 38. it will be appreciatedthat the horn 33 may be replaced by other equally suitable forms ofaudible or visible alarm indicators and that such substitution would bedeemed to fall within the scope of the present invention. The relaycircuitry associated with the stage 36 and the horn 38 comprises afail-safe type of circuitry in that it is the de-energizing of the relayG which has the effect of sounding the alarm horn.

All of the lamps 18A, 18B, and 18C in FIG. 1 are extinguished duringnormal operation throughout the systerm. The several alarm contacts 20A,29B, and 20C remain open to indicate the non-existence of dangerousvibration, temperature, pressure, or like conditions. The spacedischarge device 28 conducts anode current which energizes the relayoperating coil 'F. As a result, contacts F are closed and operating coilG is able to draw current through contacts G and F This actuation of thecoil G holds the normally closed contactsG in open position and preventsthe alarm horn 38 from producing an audible signal.

Upon the occurrence of a dangerous condition anywhere in theinstallation, a set of alarm contacts (say 20A) is caused to close. As aresult of such closure, the associated alarm lamp 18A is ionized andcaused to glow. Lamps 18A, 18B, and 180 will normally comprise panellamps which are positioned on a large panel or console to expediteinspection by supervisory personnel. When the alarm lamp 18A is ionized,a sharp pulse of charging current is'drawn by the capacitor 24A. The

resulting negative voltage drop across grid resistor 30 appears at thegrid of the tube 28 and momentarily cuts the tube 01f. Relay operatingcoil "F sufiEers an immediate loss of excitation, which causes thecontacts F to drop open. The relay operating coil G is accordinglydeprived of energizing current causing the contacts G to close. As aresult of this switching sequence, alarm circuitry 36 is caused to applypower to the alarm horn 33 for the purpose of producing an audiblesignal.

The alarm horn 38 continues to sound until the system is reset. Whenconduction through device 28 is restored, contacts F are closed.Actuation of the push button switch 34 energizes relay operating coil G,which closes the contacts G in order to maintain current through coil Gand interrupt the flow of horn power to halt the alarm.

Continuing now with the detailed description of the invention, and moreparticularly with the embodiment shown in FIG. 2, the numeral 40 hasbeen used to designate a voltage supply. The supply 40 has a groundedpositive terminal, and a negative terminal connected to one end of adropping resistor 42. The opposite end of the dropping resistor 42 isconnected to a common bus 44. The common bus 44 connects to one end of aresistor element 46A. The opposite end of resistor element 46A isisolated from ground by means of an alarm lamp 48A. A pair of normallyclosed alarm contacts 50A are connected in parallel across the alarmlamp 48A, and the common point between the resistor 46A and alarm lamp48A is connected to one plate of a capacitor 52A. The opposite plate ofthe capacitor 52A is connected directly to a common bus "54.

Directly beneath the individual circuit described immediately above, agroup of substantially identical circuits have been illustrated betweenthe common bus 44 and the common bus 54. Thus, resistor element 46B isconnected from common bus 44 to common bus 54 by capacitor 523 with thealarm lamp 483 and alarm con tacts 50B connected in shunt to ground.Similarly, resistor 46C is connected from common bus 44 to common bus 54by way of capacitor 52C, with the alarm lamp 48C and the alarm contacts500 connected in shunt to ground.

To the right, the numeral 56 has been used to identify a space dischargedevice which is normally conductive during non-alarm conditions. Thecathode of the device 56 is tied to ground, and the control gridelectrode is connected directly to the common bus 54. In addition, agrid resistor 58 is connected between ground and the grid electrode ofthe discharge device. By means of the resistor 58', the charging currentdrawn by any of the capacitors 52A, 52B, 52C when an alarm lamp isionized is capable of terminating conduction through the space dischargedevice.

The anode of the space discharge device 56 is connected to one end of arelay operating coil H. The other end of the coil H is connected toreceive operating potential from the voltage supply 6t) via a set ofnormally open contacts H A momentary contact push button switch 62 isconnected in parallel with the contacts H The switch 62 is normallybiased to open position by suitable spring means or the like, and isused to acknowledge the existence of an alarm condition and to reset thecircuitry.

The operating coil H also controls a set of normally closed contacts Hwhich in turn control an alarm circuitry stage 64. Alarm horn 66 isconnected to receive operating power from the stage 64.

In operation, the opening of an alarm contact such as 50A in response toa dangerous condition causes the immediate ionization of a correspondingalarm lamp, such as the lamp 48A. At this instant, the charging currentfor capacitor 52A drawn through grid resistor 58 causes a negativevoltage pulse to appear at the grid of the device 56 and render thedevice non-conductive.

Prior to this time, the tube has been readied for operation bydepressing the push button switch 62. Such actuation of the switch, ofcourse, places B+ potential on the anode of the device which causescurrent fiow through coil H with resultant closure of normally opencontacts H Upon the occurrence of an alarm condition, as earlierexplained, the sharp negative pulse across resistor 58 interruptsconduction in the discharge device 56. As a result of this condition,contacts H drop open which effectively removes the B+ potential from theanode of the tube. When coil H loses excitation in this fashion, thecontacts H are immediately restored to a normally closed condition whichcauses the application of power to the alarm horn 66. As the negativevoltage pulse disappears from the grid of device 56, conduction cannotbe resumed because of the absence of anode potential. The alarm horn 66thus continues to sound until B+ voltage is restored to the dischargedevice by the momentary closure of the push button switch 62 by anoperator. Once this occurs, the flow of anode current through coil Himmediately energizes contacts H in order to latch the circuit incondition for another alarm, and the circuitry is capable of alarmingagain as soon as the next set of alarm contacts open to indicate ahazardous condition.

Turning now to the detailed description of the form of the invention,illustrated in FIG. 3, the numeral 68 has been used in this figure toidentify a voltage supply which has a grounded negative terminal. Inthis form of the invention, the positive terminal of voltage supply 68is coupled to a common bus 70. The bus 70 is connected to an alarm lamp72A which in turn is connected in series with a resistor 74A. A normallyopen alarm contact 76A isolates one end of the resistor 74A from ground.

The juncture between the resistor 74A and the alarm contact 76A iscoupled to a common bus 80, by means of a capacitor 78A. Beneath thecircuit thus described, it will be noted that individual alarm lamps 72Band 72C are each connected in series with resistors 74B and 74C,respectively. .Alarm contacts 76B and 760 are used to isolatecorresponding ends of the respective resistors from ground. Capacitors78B and 78C, respectively, are used to couple the ungrounded ends of thealarm contacts 763 and 76C to the common bus 80. To the right of thisarray, the numeral 82 identifies a space discharge device having agrounded cathode as well as a grounded grid resistor 84. The controlgrid of device 82 is coupled directly to the common bus 80. Directlyabove the discharge device 82, the reference characters H and H havebeen used to indicate a relay operating coil and a set of normally opencontacts, respectively.

A momentary contact push button switch 62is connected in parallel withthe normally closed contacts H The uppermost common connection betweenthe contacts H and the switch 62 is connected directly to the positiveterminal of the voltage supply 68. To theright of the space dischargedevice, a set of normally closed contacts H is used to control theapplication of power by an alarm circuitry stage 86 to an alarm horn 90.

The occurrence of an alarm condition has the eflect of closing a set ofnormally open contacts such as the alarm contacts 76A. The alarm lamp72A is consequently ionized in order to provide a visually observablesignal and initiate a controlled transient electric current. Thetransient discharging current which is drawn through grid resistor 84thus produces a negative grid potential which terminates conductionthrough the discharge device 82.

At this point, the interruption of current through coil H withconsequent opening of contacts H and closure of contacts H causes thealarm horn 90 to receive power in the same manner as discussed inconnection with the circuitry shown in FIG. 2. I

In FIG. 4A, the negative pulse which appears at the grid of the device82 under alarm conditions is shown whereas in FIG. 4B, the positivepulse which appears at the control grid when an alarm condition clearsis illustrated.

Generally, when a given point indicates an alarm condition, theassociated alarm lamp is energized, and an acknowledgment must beeffected by depressing the push button switch 62. If any other point inthe system should alarm while the original point is returning to normal,the present circuitry actuates an alarm sequence and indicates theoccurrence of a new hazardous condition. This is because the newlyproduced negative pulse applied to the grid of the space dischargedevice cannot be cancelled out or nullified by the positive pulse whichaccompanies the clearing of the previously hazardouscondition.

FIG. 4C illustrates a composite wave form of the electrical pulseappearing at the grid of discharge device 82 when an alarmconditionoccurs-at the instant another alarm condition is clearing. Itwill be observed that the differences in timeconstants renders possiblethe initiation of a new alarm condition during the time interval inwhich positive charging current in the grid resistor indicates theclearing of the pnevious condition. The peaked negative pulse in FIG. 4Cwill be seen to appear at the control grid of the space discharge devicein full strength, although characterized by a shorter time constant onthe trailing edge.

It will now be evident that the occurrence of a fresh alarm conditionduring the restoration of a previous dangerous condition to normal willpositively actuate the alarm horn or other type indicator employed inpracticing the invention.

By means of the circuitry in FIG. 5, it is possible for operatingpersonnel to discover which of the variables in a complicated system ofinter-related variables first deviated oft-normal into a hazardous rangeof values.

In this figure, the reference numeral 92 has been used to designate atapped voltage supply provided with a grounded negative terminal. Thecommon bus 70 is coupled to a positive 200 volt tap by means of a set ofnormally open contacts H The common bus 70 is also coupled to a positive90 volt tap via a set of normally closed contacts H Contacts H and H;are of the variety known in the art as make-before-break contacts.

. The closure of a pair of alarm contacts such as 76A causes thecharging current through resistor 84 to terminate conduction in tube 82in exactly the same mann'er as described in connection with FIG. 3.However, the loss of excitation in relay operating coil H causescontacts H, to close and contacts H; to open. At this time, all otheralarm lamp circuits are rendered inoperative because of the action ofthe make-before-break contacts H and H in reducing the voltage suppliedto the remaining lamps. Until such time as relay operating coil H isre-energized by depressing the push button switch 62, all other alarmlamps are caused to remain de-energized even though one of theassociated set of alarm contacts might attempt to indicate an alarmcondition. By this means, the first cause of trouble detected by a groupof sampling instruments is indicated conclusively, with consequentopportunity for eliminating all other alarm conditions by rapidlycorrecting the first of the variables to experience the hazardous rangeof values.

While several embodiments of the invention have been shown and describedherein, it will be apparent to those skilled in the art that thestructure and arrangement of the elements of the present invention maybe changed or modified without departing from the spirit and scope ofthe appended claims.

I claim:

1.- In a system for providing an observable indication of the occurrenceof alarm conditions; a voltage supply provided with a grounded positiveterminal; a plurality of individual circuits connected each at one endto the negative terminal of said supply, each of said circuits includinga series connected resistor and capacitor with an ionizable device and anormally closed condition responsiveswitch means connected to groundtherebetween; a normally conducting space discharge device provided witha grounded cathode said device being made nonconductive in response toopening of said switch means, a grounded grid resistor, and a \gridelectrode connected in common with the other end of each of saidindividual circuits; means including relay means connected responsive tonon-conduction through said discharge device and conduction through saidionizable devices for initiating said indication of alarm conditions;and an alarm circuit operatively connected to and activated by saidrelay.

2. In a system for providing an observable indication of the occurrenceof alarm conditions; a voltage supply provided with a grounded negativeterminal and at least one positive terminal; a plurality of individualcircuits connected at one end to receive positive potential from saidsupply, each of said circuits comprising an ionizable device, aresistor, and a capacitor coupled in series, with a normally opencondition responsive switch means connected between ground and the sideof said capacitor adjacent said resistor; a normally conducting spacedischarge device provided with a "grounded cathode, said device 'beingmade non-conductive in response to closure of said switch means, agrounded grid resistor, and a grid electrode connected in common withthe other end of each of said individual circuits; means including relaymeans connected responsive to non-conduction through said dischargedevice and conduction through said ionizable device for initiating saidindication of alarm condition-s; and an alarm circuit operativelyconnected to and activated by said relay.

3. In a system for providing an observable indication of the occurrenceof alarm conditions, a power supply providing a first voltage and asecond voltage having a magnitude less than said first voltage, aplurality of individual circuits adapted to be selectively connected atone end to one of said first and second voltages, each of said circuitscomprising an ionizable device, a resistor, and a capacitor coupled inseries, with a normally open condition responsive switch means connectedbetween ground and the side of said capacitor adjacent said resistor; aspace discharge device provided with a grounded cathode, a grounded gridresistor, and a grid electrode connected in common with the other end ofeach of said individual circuits, an alarm circuit operatively connectedto said device and activated in response to non-conduction or" saiddevice, and means operatively connected to said device and responsive toconduction of said device to connect said first voltage to saidindividual circuits and responsive to non-conduction of said device toconnect said second voltage to said individual circuits.

4. A signalling system including power supply means; a plurality ofdetection circuits connected in parallel and connected to said powersupply means, each detection circuit including a switch means and anionizable device, each of said detection circuits supplying a voltagepulse in response to operation of its switch means and ionization of itsionizable device; a normally conducting device; means connecting saidplurality of detection circuits to said conducting device to apply saidvoltage pulse to said conducting device, said conducting device beingmade non-conducting at least momentarily in response to said voltagepulse; and an alarm circuit operatively connected to and activated inresponse to non-conduction of said conducting device.

5. The invention defined in claim 4, wherein each of said detectioncircuits further includes a resistor connected to limit current flowthrough said ionizable device upon ionization thereof.

6. The invention defined in claim 4, wherein each of said detectioncircuits further includes a resistor connected to limit current flowthrough said ionizable device upon ionization thereof, and furtherwherein, in each of said detection circuits, said ionizable device, saidswitch means and said resistor are connected in series across said powersupply means.

7. The invention defined in claim 4, wherein each of said detectioncircuits further includes a resistor connected to limit current flowthrough said ionizable device upon ionization thereof, and furtherwherein, in each of said detection circuits, said ionizable device andsaid resistor are connected in series across said power supply means andsaid switch means is connected in parallel with said ionizable device.

8. A signalling system including power supply means; a detection circuitconnected to said power supply means, said detection circuit includingan ionizable device, a resistor connected to limit current flow throughsaid device upon ionization, and switching means operable to connectsaid ionizable device to said power supply means to cause said device toionize; a normally conducting space discharge device; a capacitorconnected between said space discharge device and said detectioncircuit; a resistor having one end connected to said capacitor and saidspace discharge device and having the other end connected to said powersupply means, said capacitor and said last-mentioned resistor providinga voltage pulse in response to ionization of said ionizable device tocause said space discharge device to cease conducting at leastmomentarily in response to said voltage pulse; and an alarm circuitoperatively connected to and activated in response to non-conduction ofsaid space discharge device.

9. The invention defined in claim 8, wherein, in said detector circuit,said ionizable device, said resistor and said switch means are connectedin series across said power supply means.

10. The invention defined in claim} 8, wherein, in said detectorcircuit, said ionizable device and said resistor re connected in seriesacross said power supply means and said switch means is connected inparallel with said ionizable device.

11. A signalling system including power supply means providing a firstvoltage and second voltage having a magnitude less than said firstvoltage; a plurality of detection circuits, each detection circuitincluding a switch means and an ionizable device, said ionizable devicebeing ionized upon closure of said switch means provided said firstvoltage is applied to said detection circuit and remaining ionized whensaid second voltage is applied to said detection circuit while saidionizable device is ionized; a normally conducting device; a pluralityof pulse-forming circuits, one connected to each detection circuit andconnected to said normally conducting device, each pulse-forming circuitproviding a voltage pulse in response to ionization of the ionizabledevice in the corresponding detection circuit to cause said normallyconducting device to cease conduction in response to said voltage pulse;means operatively connected to said normally conducting device andresponsive to the conducting condition of said normally conductingdevice to connect said first voltage to said plurality of detectioncircuits only during conduction of said normally conducting device andto connect said second voltage to said plurality of detection circuitsonly during non-conduction of said normally conducting device, and analarm circuit operatively connected to and activated in response tonon-conduction of said normally conducting device.

12. The invention defined in claim 11, wherein each of said detectioncircuits further includes a resistor connected to 'limit current flowthrough said ionizable device upon ionization thereof.

13. The invention defined in claim 11, wherein each of said detectioncircuits furtherincludes a resistor connected to limit current flowthrough said ionizable device upon ionization thereof, and furtherwherein, in each of said detection circuits, said ionizable device, saidswitch means and said resistor are connected in series across said powersupply means.

References Cited in the file of this patent UNITED STATES PATENTS2,535,133 Hoch Dec. 26, 1950 2,556,363 Lord June 12, 1951 2,611,017Bailey Sept. 16, 1952 2,695,400 Snitjer Nov. 23, 1954 2,715,720 HenkinsAug. 16, 1955 2,724,109 Skolnick et al. Nov. 15, 1955

1. IN A SYSTEM FOR PROVIDING AN OBSERVABLE INDICATION OF THE OCCURRENCEOF ALARM CONDITIONS; A VOLTAGE SUPPLY PROVIDED WITH A GROUNDED POSITIVETERMINAL; A PLURALITY OF INDIVIDUAL CIRCUITS CONNECTED EACH AT ONE ENDTO THE NEGATIVE TERMINAL OF SAID SUPPLY, EACH OF SAID CIRCUITS INCLUDINGA SERIES CONNECTED RESISTOR AND CAPACITOR WITH AN IONIZABLE DEVICE AND ANORMALLY CLOSED CONDITION RESPONSIVE SWITCH MEANS CONNECTED TO GROUNDTHEREBETWEEN; A NORMALLY CONDUCTING SPACE DISCHARGE DEVICE PROVIDED WITHA GROUNDED CATHODE SAID DEVICE BEING MADE NONCONDUCTIVE IN RESPONSE TOOPENING OF SAID SWITHCH MEANS, A GROUNDED GRID RESISTOR, AND A GRIDELECTRODE CONNECTED IN COMMON WITH THE OTHER END OF EACH OF SAIDINDIVIDUAL CIRCUITS; MEANS INCLUDING RELAY MEANS CONNECTED RESPONSIVE TONON-CONDUCTION THROUGH SAID DISCHARGE DEVICE AND CONDUCTION THROUGH SAIDIONIZABLE DEVICES FOR INITIATING SAID INDICATION OF ALARM CONDITIONS;AND AN ALARM CIRCUIT OPERATIVELY CONNECTED TO AND ACTIVATED BY SAIDRELAY.